The family of nylons consists of
several different types. Nylon 6/6, nylon 6, nylon 6/10, nylon 6/12,
nylon 11, nylon 12, and nylon 6-6/6 copolymer are the most common. Of
these, nylon 6/6 and nylon 6 dominate the market. The numbers refer to
how many methyl units (-CH2-) occur on each side of the nitrogen atoms
(amide groups). The difference in number of methyl units influences the
property profiles of the various nylons. Moisture absorbance is
decreased due to reduced polarity with further separation and less
regular location of the very polar amide roups. Resistance to thermal
deformation is lowered due to more flexibility and mobility in these
methyl unit sections of the main chain. As these units increase in
length, making the molecules appear more like polyethylene, the
properties of the nylon shift slightly toward those of polyethylene. Not
considering the effects of moisture, Nylon 6/12 has lower modulus,
higher elongation, lower strength, lower thermal distortion temperature,
lower hardness and lower melting point than nylon 6/6. One relationship
which does not conform is price. Nylon 6/12 is more expensive than nylon
6/6. The property which gives nylon 6/12 its utility is moisture
absorption which is approximately half of that of nylon 6/6. This means
the properties are much more consistent and experience less fluctuation
due to ambient humidity levels in the end application.

Moisture absorption by nylon has been a
source of great study for many years. Although all polymers absorb some
amount of moisture, on none does it have such a significant effect as on
nylons. Table 6.1 illustrates the moisture absorption levels of various
types of nylons. (Ref 16)

Water molecules produce polar bonds with
the amide groups in the nylon molecules. Although small, water molecules
take up space and displace the nylon molecules. This results in the
nylon molecular matrix swelling. Dimensional changes of 0.7% can result
in nylon parts from the "as-molded" state to equilibrium at 50% R.H.
environments. This change occurs in approximately 150 days for a 0.0 60
inch (1.5 mm) thick part. (Ref 17) Molecular mobility is increased
through the absorption of water. The increase in spacing between nylon
molecules lowers the secondary forces allowing easier translational
motion. This is the major reasons for the change in physical properties
discussed above. There is less resistance to applied stress from the
decrease in intermolecular friction. The change in molecular mobility is
significant enough that molded nylon parts can relieve molded in
stresses as they absorb moisture. Pretty neat 'eh?

The absorption of moisture by nylon is a
completely reversible physical reaction. Drying in an oven will drive
off all but a small percentage of the water molecules which can only be
removed through dissolution of the nylon molecular matrix. The rate of
absorption/desorption varies with type of nylon as well as temperature
and relative humidity. Addition of fillers reduces the effect of
moisture both due to volume reduction of the amount of nylon polymer in
the mixture, and by sharing the attraction of the molecules somewhat
reducing polarity and the available space for moisture molecules.
Reinforcements reduce the effects more than fillers due to nylons strong
affinity for reinforcement. In addition to the mechanisms which take
place with fillers, the adhesion of the nylon molecular matrix to
dimensionally stable reinforcements is stronger than than polar bonding
of the water molecules and it dominates. Kinda like my ex...

Another area where moisture has
significant effects on nylons is in processing. Heated to molding
temperatures while wet (ie., >0.2 % water) will result is hydrolytic
degradation and a significant loss of physical properties. (Hydrolytic
degradation is a chemical reaction which occurs at high temperature with
some polymers in the presence of water. It causes primary bonds in the
molecular chains to be severed thus reducing molecular weight.) Over
drying (ie., <0.08% water) will remove the plasticizing effect of the
water molecules and make the resin very viscous and hard to flow. The
plasticizing effect in processing has to do with mobility and relative
spacing of the nylon molecules, the same influence as on physical
properties. This low level of moisture does not cause significant
degradation during processing. The absorption of moisture by nylon must
be considered in mold making. The shrinkage factor used in designing the
mold must take the the potential for change in post molded dimensional
into account. Although moisture causes problems in working with nylons,
it does contribute to: better dyeability, toughness, softness and
greater flexibility in nylon parts.

Another dominant feature of nylons is
crystallinity. As with most crystalline polymers, the molecular chains
are uncluttered by large substituent groups. They are flexible and
regular in group spacing and crystallize readily. As with acetals, this
crystallinity is responsible for properties of wear resistance, chemical
resistance, thermal resistance, and unfortunately, higher mold
shrinkage. The overall excellent property profile of nylons results in
their probably having the most diverse range of applications of all
thermoplastic polymers. Now let's talk about cutting nylon.

TIPS FOR MACHINING
NYLON

STORAGE
Nylon has a high coefficient of thermal expansion (about three times
that of aluminum) and low heat conductivity. Make sure that it has been
exposed to normal room temperature for several hours before it is
machined into finished parts.

SAWING
Nylon can be easily sawed on standard metal working equipment. Wood
working equipment may be suitable but the high cutting speeds may cause
excessive heat build-up. A blade that has been used for cutting metal is
usually not sharp enough for nylon. Use a new coarse tooth blade with
good set. Coolant may be used to control heat buildup and to prevent
melting the nylon.

HOLDING
Keep in mind that nylon is not as strong as metal and can be deformed by
improper chucking methods. On small accurately sized rod, use standard
spring collets. On larger parts, use a 6-jaw universal chuck instead of
a conventional 3-jaw chuck to distribute the holding force more
uniformly. For thin walled tubular shapes, machine soft jaws so that the
part is almost entirely confined.

TURNING
Satisfactory finishes can be easily obtained on nylon over a wide range
of surface speeds. Use tools that are honed sharp and have high rake and
clearance angles, to minimize cutting force and reduce heat build-up.
Chips will be continuous and stringy. They should be directed away from
the cut and prevented from winding around the workpiece. Coolants are
generally not necessary for lathe work unless there is excessive heat
build-up.

MILLING
Milling cutters should be honed sharp and should have high positive
cutting angles. Care should be used in clamping the part to prevent
distortion. Double-faced pressure sensitive tape can be used to hold
down flat parts. Cutting speeds and speeds will be determined by the
finish required and will be limited by heat build-up.

DRILLING
Use conventional twist drill or flat type drills. Polished flutes will
aid in the removal of chips. Do not use metal cutting reamers with
nylon. They do not cut freely enough. Drill small holes to size in one
operation. Rough drill large holes and finish by single point boring.

THREADING
Use only sharp taps and dies on nylon parts. Don't use tools that have
been used to cut metal. H5 or even larger oversized taps may be required
because a threaded hole in nylon closes in when the tap is removed.
Threads to close tolerances can be easily single point chased.

GRINDING
The large amounts of heat generated by grinding, together with the low
heat conductance of nylon, usually dictate that liberal amounts of
coolant he used in most grinding operations. Thru-feed centerless
grinding of long, flexible parts of nylon can be easily accomplished,
and tolerances as close as .0005" are possible. Cylindrical grinding on
nylon is usually not required because it is easy to get good finishes
and close tolerances on a lathe. Surface grinding of nylon is usually
not necessary. If a flat surface with close tolerances and good finish
are required, the best approach is fly cutting in a milling machine. No,
not cutting a fly on your milling machine, FLY cutting.

STAMPING
Thin pieces may be stamped with standard equipment. Thick sections will
require high shear angles if good edges are needed. Steel rule dies may
be used for some parts.

MEASURING
Use ordinary measuring equipment. However, use a light touch because the
material is not as hard as metal. A micrometer anvil can deform a nylon
surface as much as several thousandths. Homemade, soft plug and ring
gauges are useful on thin walled parts. If extremely close tolerances
are involved, make SURE any temperature changes that the part will see
are taken into account.

PROPERTIES

A.S.T.M Test Method

NYLON

NYLON

NYLON

NYLON

TYPE 6

TYPE 66

TYPE 612

CAST TYPE 6

Specific Gravity

D792

1.12 - 1.14

1.14 - 1.1

1.06

1.15

Water Absorption Method A

D570

2.9

1.24

0.25

-=-

Tensile strength at yield, 1000
psi

D638

9.4

12

8.8

11 - 14

Elongation at yield, %

D638

25

>150

7

10

Elastic Modulus in Tension, 10~5
psi

D638

-=-

4.4

-=-

3.5 - 4.5

Flexural Strength at yield, 1000
psi

D790

NO YIELD

16

NO YIELD

16 - 17.5

Elastic modulus in flexure, 10~5
psi

D790

1.50

4.1

2.95

-=-

Rockwell Hardness (Method A)

D785

R104

88

R114

R112

Izod impact strength, ft-lb/in.
notch 1/8 in. speciman

D256

2.2

1.2

1.5

-=-

Deform. under load(2000 psi;
122f), %

D621

-=-

0.8

1.6

0.5 - 1.0

Deflection temperature, F at 66
psi fiber stress

D648

340

450

356

400

Max recommended service Temp., F
continuous use

-=-

175

270

290

200 - 225

Coeff. of Linear Thermal
Expansion, F

D696

4 x 10~5

4.5 x 10~5

5 x 10~5

5.0 x 10~5

Underwriters' Lab Rating (Subj.
94)

-=-

HB

V - 2

V - 2

-=-

Dielectric strength, v/mil, short
time

D149

-=-

555

650

500

Dielectric constant at 60 Hertz

D150

7.2

4.0

4.0

3.7

Dielectric constant at 1
MegaHertz

D150

3.7

3.5

3.5

3.7

Dissipation factor, at 60 Hertz

D150

-=-

0.02

.02

-=-

Dissipation factor, at 1
MegaHertz

D150

0.12

0.03

0.2

-=-

Volume resistivity, ohm-cm

D257

10~12

10~15

10~15

-=-

Arc resistance (SS Electrode),
sec.

D495

-=-

123

-=-

-=-

These values are representative of those obtained under
standard ASTM conditions and should not be used to design parts which
function under different conditions. (Somebody made me say that...)